Many systems such as cooling towers, fluid coolers, and condensers operate using make-up water that contains minerals. Pure water (mineral free) is lost from the system via evaporation resulting in the mineral concentration in the remaining water increasing. Periodically some of this highly concentrated water is removed from the system via blowdown and replaced with lower mineral content make-up water. This method results in a fairly consistent level of minerals in the recirculating water. The higher the concentration of recirculating minerals relative to make-up, the less amount of make-up water is used by the system. To allow proper operation at high mineral levels, chemical inhibitors are often added to the system to minimize the corrosion and scaling that would otherwise occur. Typically, these chemicals are in liquid form and are pumped into the system based on some estimate of the loss of chemicals by blowdown.
It is known that solid chemicals offer benefits over liquids. Much fewer pounds of solids are needed, there is no danger of a concentrated liquid spill during transportation and storage, and handling is simplified. The problem is that it has been difficult to accurately feed a solid inhibitor into a cooling system. Several different approaches have been tried. One such approach is exemplified by U.S. Pat. No. 6,418,958 and U.S. Pat. No. 6,820,661. These patents describe inventions where the solid chemical is first dissolved into a day-tank and then the liquid is pumped or aspirated from the day-tank into the system. These inventions require somewhat elaborate methods to dissolve the solid and still require pumps or other methods to get the liquid into the cooling system. These day-tank feed methods typically require that make-up water be used, not recirculating water, with the added risk of backflow contamination of the potable water system.
Another technique is to make a timed-release solid that will release at a more or less steady rate. An exemplary example of this technique can be found in U.S. Pat. No. 7,883,638 where solid chemicals are coated with an insoluble and semi-permeable membrane. The membrane is designed to allow the consistent release of chemicals. When the chemicals are fully released, the coating remains as a disposable waste. This technique can be very wasteful of chemicals as the release rate must be designed for the maximum amount of short-term blowdown on the system while most of the time the system is operating at less than full capacity. Also it is often difficult to run the chemicals until they are fully exhausted.
Many inhibitors are difficult to measure with a field test and often an indicator-chemical is analyzed to determine the concentration in the cooling system. U.S. Pat. No. 4,783,314 describes how an indicator-chemical, particularly one that fluoresces, can be added in a specific proportion to the liquid inhibitor and then the concentration of the inhibitor in the system can be determined by measuring the concentration of the indicator-chemical. Since the chemical fluoresces, this measurement can be determined by automated techniques. U.S. Pat. No. 4,992,380 describes how this technique could be used in a feedback loop to maintain a desired level of a liquid inhibitor in the cooling system.